the phenomena of taboo trade-offs, then it provides a novel prediction: taboo trade-offs will prevail precisely in situations where there is large but infrequent temptation to defect and defection is harmful, such as selling a child, betraying a country, or sleeping with someone for a million dollars. It remains to be shown that taboo trade-offs demonstrate this characteristic. It also provides an important policy prescription regarding policies forbidding taboo trade-offs, for example, the ban on euthanasia: such policies are socially suboptimal, since the benefits of cooperating without looking accrue to the individuals who advocate them, but the costs are borne by society. Finally, our model offers an explanation for emotions such as love, and can thus be thought of as a formalization of Frank (1988). Love has the property that we behave altruistically towards our partners regardless of what temptations arise [30], as illustrated by the wedding vow, “for better or for worse, for richer, for poorer, in sickness and in health.” For example, love causes individuals to ignore other potential mates, even if those mates are better than one’s current mate, as Shakespeare’s Juliet did when her love for Romeo led her to rebuff the advances of the otherwise-more-suitable Paris. Why does love have this property? Our model suggests that those in love will more often be chosen as long-term partners. If this argument in fact underlies love, then it is crucial that love is observable, and that it necessarily cannot be displayed while still attending to costs. There is evidence consistent with this: related emotions are observable [31], cannot be faked [32], and are relied upon by partners when choosing whether to cooperate [33]. There is also reason to believe love and related emotions would be hard to fake, given their autonomic origins, and the costs of placing their activation under conscious control [30, 28]. However, it remains to be shown that love in particular has the